The growing emphasis on competitiveness and efficiency in the industrial marketplace has brought about dramatic changes in the methods by which a variety of production tasks are performed. An ever increasing number of robotic systems are finding a place in the production cycle of nearly every conceivable good produced. From the handling of raw materials, to the production of goods themselves, to storage and shipment, the flexibility of robotic systems has made an impact and created a new standard of efficiency.
Despite the phenomenal progress of robotic systems within the last twenty years, the potential for further improvement and growth is seemingly unlimited. One area of paramount importance is the design of end effectors. End effectors are usually understood to be that portion of a robotic system which interacts with the environment. Since most industrial robotic systems may be loosely classified as "arms", end effectors may be said to represent robotic "hands". Generally, end effectors are independently controllable and, as such, are used to manipulate a tool, or may be used to manipulate a work piece, part, or material. An end effector is also the ideal tool to gain information about an environment and the objects to be manipulated. Ideally, an end effector should be able to perform a variety of tasks by simply changing the programming instructions which control its movements. In practical terms however, this is not feasible because end effectors operate within a large range of loading and spatial precision requirements, as well as a variety of environmental conditions. As the complexity of robotic systems and the tasks they are asked to undertake grows, the improvement of the devices which allow them to affect and learn about their environment increases in importance.
As the science of robotics progresses, truly programmable systems must be created which can approach an unknown environment and undertake a variety of tasks. Such systems will allow exciting new applications in outer space, hazardous environments, military applications, and undersea exploration. Robotics will thus move from the industrial sphere to the more general goal of reducing the labors and dangers encountered in performing a variety of tasks within an undefined environment.
Existing end effectors can be divided into two broad categories. The end effectors in the first of these categories may be described as complex multi-fingered hands. These are characterized by the ability to undergo precise fingertip movements, and have been successful in achieving an ability to handle small objects with precision, dexterity, and controlled grasping force. This category includes attempts to design anthropomorphic hands which seek to replicate the movements of the human hand. However, end effectors in this first category suffer from limited strength, fragile construction, and undue complexity-drawbacks which reduce their effectiveness as useful equipment in demanding environments. The second class of end effectors comprises the well known simple grippers found in most industrial robotic systems, as well as somewhat more complex devices. These devices are strong and robust, capable of carrying useful payloads and performing reliably. However, the structural and control system simplicity inherent in these designs necessarily limits their adaptability to a variety of tasks; most are considered useful for a single purpose, such as welding, painting, drilling, part insertion, or material transfer. The cost incurred in the design and construction of a specialized end effector, and the inefficiency involved in changing between them, frequently presents a major drawback to the optimal implementation of robotic systems.
Examples of complex multi-fingered hands are described in Salisbury, J. Kenneth, Robot Hands and the Mechanics of Manipulation, The MIT Press, Cambridge, Mass. (1985); and Jacobsen, S., et al. "Design of the Utah/MIT Dexterous Hand", Proceedings of the IEEE Conference on Robotics and Automation (1986); both of which are incorporated herein by reference.
The Salisbury hand contacts an object with three fingertip contacts and relies on frictional constraints and force feedback to provide a stable grasp. The configuration is derived from the results of a numerical analysis of possible hand designs. From these results, a hand comprised of a combination of three fingers with three rotational joints per finger was developed. This device has the capability to perform fingertip manipulation of small objects--a one inch sphere was chosen as a typical example--and can be programmed for small parts motion.
The Utah/MIT hand essentially duplicates the human hand. Although it has only four fingers, each finger is designed with four joints and anthropomorphic geometry. This design is extremely costly and requires very complicated control techniques and hardware. Thus, this design is limited to an advanced research environment.
The second category of end effectors, simple one and two degree-of-freedom grippers, are the most commonly used end effectors in both research and industry. A description and illustrations of examples of these grippers may be found in Coifett, P. and Chirouze, M.; An Introduction to Robot Technology, McGraw-Hill (1982), which is incorporated herein by reference, particularly at pp. 159-163. Although these grippers have limited versatility, they are generally robust and economical. In practice, object variation requires a specialized end effector for each application.
In an effort to reduce the complexity of the control systems of robotic fingers, as well as reducing the number of actuators, designs have been proposed which feature joints which are rigidly coupled. Rigid coupling between joints defines a single set of joint angles for each actuator displacement. For example, if two joints are coupled by pulleys with radii of r.sub.1 and r.sub.2, the joint displacements .phi..sub.1 and .phi..sub.2 are defined by the relation: ##EQU1##
An example of a finger having coupled joints may be found in U.S. Pat. No. 4,865,376--Leaver et al., which is assigned to the same entity as the present application and is incorporated herein by reference. Leaver et al. discloses a three-joint finger having two actuators, as well a proposed method of representing and optimizing fixed, rigid couplings between joints by using a matrix method of representing the cable routings used to create such couplings. The joints of Leaver maintain a known ratio between the joints and thus are limited in the class of objects which may be enclosed in their grasp.
The placement of fingers relative to a palm can also play a role in the manipulation of objects, as set forth in U.S. Pat. No. 3,866,966--Skinner, II. The three fingered hand disclosed by Skinner, II uses a palm structure which allows the fingers 12,12a,12b to be rotated about an axis which is coincident with the base link of the finger. Thus the palmar surfaces of the fingers, those which generally face toward the palm and together with it close around an object, may be "turned" relative to each other, but the base of the finger remains fixed in a single position relative to the palm. Although Skinner thus provides a rotational degree of freedom for each finger, it is not possible to translate position of the fingers relative to the palm, thereby allowing a variety of cooperating and/or opposing orientations to be achieved.
Thus, it can be seen that there exists an unfulfilled need for a device which combines the simplicity, robustness, and ease of control characteristics found in simple grippers, while also possessing the versatility and dexterity of the more complex multi-fingered hands. Therefore, it is an object of the present invention to provide a class of end effectors possessing greater degrees of freedom and versatility than existing grippers, while further providing the robustness, utility and economical construction not found within available multi-finger hand designs.
It is desirable to provide designs for robotic hands which possess a sufficient number of degrees of freedom between the fingers and palm to enable a variety of grasps to be accomplished. The actuators for each joint should be placed at or near each joint for simplicity and to reduce the complexity and inefficiency in power transmission. It is therefore advantageous to reduce the number of actuators needed to effect a particular grasp geometry. Accordingly, it is an object of this invention to provide palm/finger/actuator configurations which maximize the variety of grasps and dexterity available, while minimizing the number of actuators. It is a further object of the present invention to allow the fingers used in the hand of the present invention to be capable of being positioned in a variety of orientational positions relative to the palm to which they are attached.
In order to execute a firm grasp, an articulated member such as a robotic finger should be capable of being "wrapped" around an object. It is further desirable that this function be executed with little prior knowledge of the shape of the object being grasped and without actively controlling the joints in a servo loop. Thus, a further object of the present invention is to provide articulated members which comply with the shape of an object to provide a firm, enveloping grasp. It is also an object of the present invention to provide articulated members which rely on mechanical intelligence to execute this function, rather than requiring a series of joint actuation control commands.
The attempted grasp of an object may fail due to the varied spatial orientation of the object affecting the stability of the grasp. It is a further object of the present invention to provide apparatus for manipulating an object fitted with tactile sensors which can determine grip stability. It is yet another object of the present invention to present methods whereby the information collected by tactile sensors may be utilized to adjust the orientation of the apparatus to achieve a stable grasp.
It is also an object of certain embodiments of the present invention to provide fingers which possess a degree of compliance upon executing a closing grasp, yet remain rigidly resistive to the opposing force they encounter.